Project 6, APOBEC/AID family of proteins: In the absence of the HIV accessory protein Vif, virionencapsidated APOBEC3G (A3G) causes extensive C-to-U mutations, thereby damaging newly synthesized minus-strand viral DNA). it also physically blocks RTxn. These actions render the virus noninfectious. To understand A3G's activities at the atomic level, we will determine the structures of several proteins of the APOBEC/AID family, using standard solution NMR and X-ray crystallography approaches (see NMR and X-ray Core descriptions). All APOBEC/AID proteins are cytidine deaminases that contain Zri binding clusters and exhibit a high degree of primary sequence similarity, for one-domain as well as two-domain variants. Among all the APOBECs, A3G has received the most attention, given its ability to restrict HIV in a delta Vif background. The monomeric as well as dimeric models of A3G were created based on an A2 Xray structure that exhibited a tetrameric arrangement in the crystal in striking contrast to the X-ray findings, we recently uncovered that APOBEC2 (A2) is monomeric in solution, and we have prepared labeled protein for NMR structure determination. The anticipated structure will yield important information with respect to the validity of using A2 as a surrogate model for A3G. The great importance of high-resolution structural information for A3G compels us to continue our endeavor towards obtaining a bonafide A3G structure, however, we also will study the related APOBEC3A (A3A) protein, characterizing its structure and enzymatic activity at the atomic level. Altogether the A2, A3A and, possibly, A3G structures should fill a conspicuous gap in our understanding of cytidine deamination in the context of HIV biology.